1. Field of the Invention
The invention relates to a microphone array in a housing receiving sound via guide tubes.
2. Description of the Related Art
A typical microphone array includes a number of microphones disposed in tandem. A simple example is shown in FIG. 1, wherein the microphone array 10 includes two microphones 11 and 12 placed side by side. Directivities of the microphone array 10 can be achieved by manipulating the signal received by the two microphones 11 and 12. Assuming the two microphones 11 and 12 are omni-directional and having the same characteristics, the directivity of the microphone array 10 depends on vector dω from one microphone 11 to the other microphone 12.
The above-mentioned microphones 11 and 12 are placed in open space for achieving the directivity. Most electronic devices (cellular phones, personal digital assistants, etc.), however, have plastic or metal housings, which are acoustic isolators. Acoustic isolators block audio signals increasing difficulty in placing microphones. Furthermore, in modern devices, the majority of electronic elements including microphones are surface-mounted on printed circuit boards (PCBs), thus limiting the directivity of the microphone array. As illustrated by FIG. 2, microphones 11′ and 12′ are disposed in a housing 20. The housing 20 acts as an acoustic isolator preventing the microphones 11′ and 12′ from receiving external sound. Furthermore, the distance d1 between the microphones 11′ and 12′ on the PCB 21 is limited by the available space on the PCB 21 and in the housing 20, and generally is less than the desired distance in design (d1<d). The direction of the microphone array, as designated by vector {right arrow over (d1)}, is always parallel to the PCB 21. Such a direction, however, is generally not toward the sound source during operation of the electronic device.